1. Field of the Invention
The field of the present invention is that of turbojet engines and of the components that join their various parts together. It relates more particularly to the components that ensure the mechanical integrity of the fan duct of bypass turbojet engines.
2. Description of the Related Art
A propulsion engine, such as a turbojet engine, may be mounted at various locations on the aeroplane, by attaching it to a strut or pylon belonging to the structure thereof. It may thus be suspended under the wing, fixed to the fuselage, generally at the rear, or mounted in the tail using appropriate means of attachment. These means of attachment have the function of transmitting mechanical load between the engine and the structure of the aeroplane. The loads that have to be taken into consideration are notably the weight of the engine along the vertical axis Z, its thrust along the axis X of the engine, and lateral aerodynamic loading along the transverse axis Y. The loads that have to be transmitted thus include reacting the rotational torque about the axis of the engine. These means have also to absorb, without transmitting it to the strut, deformation experienced by the engine during the various phases of flight as a result for example of dimensional variations caused by thermal expansion or contraction.
One method of suspension, for example, in the case of a turbofan engine, is to attach the engine to a strut belonging to the structure of the aeroplane using a front suspension or fastener and a rear suspension or fastener. The front suspension is fixed in particular to the intermediate case downstream of the fan case and the rear suspension is fixed to the exhaust case for the primary flow. These two parts constitute the structural elements of a turbomachine on which all load is reacted.
Modern turbojet engines are bypass turbomachines with high bypass ratios, the bypass air being compressed by a single compressor stage known as the fan stage. On leaving this stage it is guided along a duct directly into a nozzle to contribute to the thrust produced by the engine. It thus flows between the main spool of the engine, delimited by cases, and a cold flow duct (more usually known as the outer fan duct, or OFD). For weight saving reasons, this duct is commonly made of composite. This duct is attached to the engine via connections at its two longitudinal ends, a first fixing being at the upstream end to the case surrounding the fan and a second at the rear end to a mounting ring borne by the exhaust case.
The connection between the mounting ring and this exhaust case is generally achieved by means of arms that pass across the fan flow. In more recent embodiments, it may also be afforded by profiled connecting rods attached firstly to the fan duct and secondly to the exhaust case, and this allows a significant reduction in the mass devoted to this connection. In this scenario of connecting rod attachment, the connection is performed by a latticework of connecting rods, generally six or eight of these, which are attached at six or eight points to the mounting ring as shown in FIG. 2 or 3. An example of how a fan duct is attached using six connecting rods is, moreover, described in the applicant company's patent application FR 1153947 filed on 9 May 2011.
Such an assembly is deliberately statically indeterminate, notably to ensure that the turbomachine is held optimally in position by imposing a limit on the degree to which it can become distorted. The downside of such a choice is that the system is difficult to fit and that devices for adjusting the length of the connecting rods need to be provided, otherwise mounting becomes impossible. Thus it is necessary to provide a system for adjusting the position of the ball ends of the connecting rods as a function of the length of these rods, or alternatively for adjusting the position of the pins mounted on the ring, in order to take account of the manufacturing tolerances on each of the connecting rods and on the yokes of the exhaust case and of the ring.
In the prior art, the connecting rods are generally adjusted using an eccentric bushing incorporated into an eye of the connecting rod, and the turning of which makes it possible to adjust the length of the connecting rod to suit the desired distance. In these embodiments, the axis of the hole in the connecting rod is not necessarily aligned with the axis of the connecting rod, because of the eccentric. Under load, this generates an undesirable moment at the attachment of the connecting rod and the bushing therefore has a tendency to turn. Further, instances of seizure associated with the use of these eccentric bushings have been observed in service. Finally, the possibilities of adjustment offered by this device are still limited.
One solution envisaged has been to use an eccentric at the ring end and to add a system for preventing the bushing from turning. However, such a system is difficult to employ and represents a not inconsiderable additional weight.